Silicone elastomers have been widely valued for their resistance to moisture and their high and low temperature stability. Silicone elastomers have been developed which exhibit excellent handling characteristics, as in U.S. Pat. No. 2,938,009 (Lucas), mechanical properties, as in U.S. Pat. Nos. 3,635,743 (Smith) and 3,847,848 (Beers), and solvent resistance, as in U.S. Pat. No. 4,029,629 (Jeram). All of the above patents are incorporated herein by reference. These characteristics are accomplished through the use of treated fillers, usually treated finely divided silica, or by modifying the curable siloxane polymers.
Treating silica fillers with silicone treating agents has been proposed, see U.S. Pat. No. 4,355,121 and U.S. Pat. No. 4,469,522, but implementation of these concepts has been resisted because of the toxicity of the treating agents (i.e., fluoroalkyl functional cyclic polysiloxanes) and the substantial modifications of equipment their use would entail.
Another method for treating silica fillers with silicone treating agents is disclosed in U.S. Pat. No. 4,529,774 (Evans et al.). Therein silica fillers are exposed in the vapor phase to the direct hydrolyzate of diorganosilanes at temperatures of from 240.degree. C. to about 310.degree. C. These direct hydrolyzates are compounds comprising cyclic and linear diorganopolysiloxanes. The high temperatures utilized make both the equipment and energy necessary to carry out this method costly. Additionally, reproducibility by this method is poor.
A method has been discovered by which finely divided silica fillers can be treated with diorganopolysiloxanes at lower temperatures and subsequently at lower cost. Furthermore, such method calls for process conditions that allow for the overall simplification of producing treated silica fillers.
Accordingly, it is an object of the present invention to provide a method for treating silica filler which allows for the use of lower reaction temperatures and improved reproducibility.